There’s a tricky chemical trade-off at work in our skies. As greenhouse gases provide their famous warming effect to Earth’s surface, aerosol pollution in the atmosphere actually partly counteracts it. Aerosols are tiny particles suspended in the air, both natural and industrial, including sea salt, mineral dust, ash, soot, sulphates, nitrates, and black carbon. They hang around in the air for around 10 days, scattering and absorbing radiation from the sun. Aerosols also provide nuclei for water droplets, boosting cloud formation, thus decreasing the amount of energy reaching the ground and providing a net cooling force. In short, greenhouse gases warm the surface; aerosols cool the surface.

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An aerosol is a suspension of fine solid particles or liquid droplets in a gas. Examples are clouds, and air pollution such as smog and smoke.

A greenhouse gas (sometimes abbreviated GHG) is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapour, carbon dioxide, methane, nitrous oxide, and ozone.

This trade-off creates an interesting dilemma in parts of Asia, considering China and India’s coal burning creates an aerosol problem far worse than the United States or Europe ever had — even before the Clean Air Act of 1970 (in America) and before the collapse of the Eastern Block’s dirty economy reduced aerosol emissions dramatically. In principle, if China and India were to begin fixing their aerosol problem, which kills hundreds of thousands every year, they might actually contribute to global warming (if they don’t also cut greenhouse gas emissions); the cooling effect of aerosols would be removed, leaving greenhouse gases to warm the globe unimpeded.

"It’s about quantification," says Marshall, the Cecil and Ida Green Professor of Oceanography. "One of the biggest challenges of climate science is to quantify the relative role of aerosols and oceans in cooling the planet’s surface vs. the greenhouse gases warming it. Just knowing the trade-off happens isn’t enough to make useful predictions and inform policy makers."

Rong Zhang, lead author and oceanographer in the Geophysical Fluid Dynamics Lab at NOAA, became curious about a climate model used by a United Kingdom research group at the Met Office Hadley Centre. The group argued in the journal Nature that anthropogenic aerosols are a prime driver of 20th-century North Atlantic climate variability, even influencing peaks in hurricane activity and the Sahel drought. Zhang and her colleagues decided to look deeper.

They found that the simulations of the group’s HadGEM2-ES climate model could not replicate the actual observations of the North Atlantic in the 20th century: Substantial warming trends in the heat content of the upper ocean have been observed in most ocean basins since 1955, yet the group’s model ocean is much colder. Zhang’s team discovered that the discrepancy in ocean heat content is influenced and largely caused by modeled aerosol cooling effects on ocean temperature.

Indeed, aerosols, via clouds, reduce energy reaching the ocean, contributing a net cooling effect to both the surface and subsurface ocean temperature. In the U.K. model, the aerosol’s cooling effect is so strong it even cancels out any greenhouse gas-induced warming. However, actual observations show a much warmer ocean.

One very difficult problem with modeling the cloud-mediated aerosol effect is cloud formation itself. The small-scale process of cloud formation in nature is notoriously difficult to model. "The HadGEM2-ES model and most state-of-the-art climate models used for the Fifth Assessment Report of the Intergovernmental Panel on Climate Change only have about 100-km horizontal resolution in the atmosphere, which is not enough to resolve the cloud process," Zhang says.

It’s not just the Met Centre that’s having trouble modeling aerosol effects on climate variability — everyone is. "We simply don’t have sufficient constraints on the history, spatial distribution and microphysics of anthropogenic aerosols," says Gavin Schmidt, climatologist and climate modeler at the NASA Goddard Institute for Space Studies (GISS) in New York.

Thus we are left with the knowledge that action to address the greenhouse gas versus aerosol problem will likely have to proceed without precise quantification of their respective temperature effects on climate.

One geoengineering project, currently under consideration, would involve the intentional injection of aerosols, specifically sulphate particles, into the stratosphere (essentially replicating the effect of volcanic eruptions). "If you put aerosols into the stratosphere," Marshall says, "it will certainly cool the planet’s surface, but there will be other consequences, many unforeseen, for example, to the hydrological cycle and polar caps." What we do not know can harm you.